1. Field of the Invention
The present invention relates to a seat reclining apparatus.
2. Description of Related Art
A seat reclining apparatus in which a base plate and a ratchet plate (ratchet) that are rotatable relative to each other about a relative rotation axis are fixed to one and the other of a seat cushion side frame and a seatback side frame, respectively, is known in the art as a seat reclining apparatus installed onto a vehicle seat. An internal gear which extends circumferentially about the relative rotation axis for the base plate and the ratchet plate is formed on the ratchet plate, and a plurality of lock members, each of which has an outer toothed portion engageable with the internal gear of the ratchet plate, are supported by the base plate to be movable between an engaged position in which the outer toothed portion is engaged with the internal gear and a disengaged position in which the outer toothed portion is disengaged from the internal gear. Operating an operating mechanism (cam mechanism) causes each of the plurality of lock members to move between the engaged position and the disengaged position, thus allowing the seatback to be fixed at an arbitrary position when the seatback is in a seating position, whereas each of the plurality of lock members is held in the disengaged position by an unlocked-state forcibly holding mechanism when the seatback is positioned between the fully-forward tilted position and the first-stage locked position. This structure is disclosed in Japanese Unexamined Patent Publication No. 2003-9978.
This unlocked-state forcibly holding mechanism has been structured such that at least one of the plurality of lock members is provided with a forced lock release projection and the ratchet plate is provided with a forced lock release surface which engages with the aforementioned forced lock release projection to hold the aforementioned at least one lock member in the disengaged position when the seatback is in between the fully-forward tilted position and the first-stage locked position (Japanese Unexamined Patent Publication No. 2003-9978).
However, if the lock member(s) provided with the forced lock release projection is positioned above the relative rotation axis while another lock member(s) provided with no forced lock release projection is positioned below the relative rotation axis, there is a possibility of a half-locked state occurring as a result of the lock member(s) having no forced lock release projection which is held in the disengaged position by the unlocked-state forcibly holding mechanism moving in the downward direction (a direction to be engaged with the internal gear of the ratchet plate) due to some reason (e.g., due to an operation of the aforementioned operating mechanism or differences in dimensions of the lock members and the guide grooves). The aforementioned half-locked state is a state in which the outer toothed portion of the lock member(s) that is movably held by the base plate is half-engaged with the internal gear of the ratchet plate, thus making it difficult to make an adjustment to the seatback or causing an incomplete locked state of the seatback.